WASHINGTON (Reuters) – President Barack Obamaâ€™s campaign to curb gun violence after the Newtown school massacre was dealt a crippling blow on Wednesday when the Senate rejected a plan to expand background checks for gun buyers.

Despite emotional pleas from families of victims of the shootings in Newtown, Connecticut, and broad public support, the plan to extend background checks for sales made online and at gun shows failed on a 54-46 vote, six short of the 60 votes it needed to clear a procedural hurdle in the Senate.

It was a stark reminder of the gun cultureâ€™s hold on America – or at least its politics – and a display of how each party has used Senate rules to effectively block legislation even when it has the support of the majority.

At the White House, a visibly angry Obama said he sympathized with Americans who were trying to make sense of it all.

â€œThis was a pretty shameful day for Washington,â€ said Obama, flanked by Newtown families and former Representative Gabrielle Giffords, who was severely wounded in a 2011 mass shooting in Arizona.

â€œI see this as just Round One,â€ Obama said. â€œSooner or later, we are going to get this right. The memories of these children demand it.â€

The plan by Democrat Joe Manchin of West Virginia and Republican Pat Toomey of Pennsylvania to expand background checks represented Obamaâ€™s best hope to pass meaningful gun-control legislation after the killings of 20 children and six adults at Newtownâ€™s Sandy Hook Elementary School.

Other measures backed by the president – including a proposal to ban rapid-firing â€œassaultâ€ weapons like the one used in Connecticut and a limit on ammunition clips – also failed in a series of Senate votes that reflected senatorsâ€™ reluctance to be seen as undermining the constitutional right to bear arms.

The votes also showed the enduring political power of gun-rights defenders and the National Rifle Association, the nationâ€™s largest gun lobby.

â€œOur hearts are broken. Our spirit is not,â€ Mark Barden, whose son was killed in Newtown, said at the White House after the vote as Obama looked on.

Barden vowed that his group of Sandy Hook victimsâ€™ family members would continue to seek â€œcommon-sense solutionsâ€ to gun violence.

â€˜SHOW SOME GUTSâ€™

The votes in the Democratic-led Senate on Wednesday were the culmination of weeks of intense negotiations and lobbying over the proposed gun restrictions, and signaled the likely demise of the biggest package of gun legislation in Congress in two decades.

The political momentum for new gun-control laws had dissipated after Decemberâ€™s shooting. Opponents criticized the proposals as government overreach that would infringe on citizensâ€™ gun rights under the U.S. Constitutionâ€™s Second Amendment. The NRA mounted an aggressive lobbying effort that Obama said amounted to a series of â€œliesâ€ and â€œscare tacticsâ€ about the potential impact of gun legislation.

â€œExpanded background checks would not have prevented Newtown,â€ Republican Senator Charles Grassley of Iowa said on Wednesday in opposing the Manchin-Toomey plan.

Obama said senators were too worried that a â€œvocal minorityâ€ of gun owners would make them pay in the next election for their vote to support the gun-control amendments.

When the vote totals on the background checks plan were announced by Vice President Joe Biden, who presided over the Senate tallies, a spectator in the chamberâ€™s gallery shouted, â€œShame on you!â€

Feinsteinâ€™s proposal to ban the sale of assault weapons drew support from just 40 members of the Senate, where Democrats control 55 of the 100 seats. The proposal to limit the size of ammunition magazines drew only 46 votes. All of the Senate amendments required a 60-vote threshold to clear procedural hurdles.

Several Senate Democrats are from states where hunting and gun ownership are a part of the culture, and any vote against gun rights increases the risk of facing a tough re-election fight against a well-funded Republican who will not compromise on gun rights.

â€˜EVERY MEANS POSSIBLEâ€™

Four Democrats who will face re-election in conservative, gun-friendly states opposed the Manchin-Toomey background checks amendment – Heidi Heitkamp of North Dakota, Mark Begich of Alaska, Mark Pryor of Arkansas and Max Baucus of Montana.

Senate Democratic leader Harry Reid supported the measure, but changed his vote after it was apparent it would lose in order to preserve his option to bring the measure back up in the Senate.

Four Republicans backed it: Toomey, Susan Collins of Maine, John McCain of Arizona and Mark Kirk of Illinois.

Supporters promised political payback for foes of the amendment. Giffords said in a joint statement with husband Mark Kelly that the Senate â€œignored the will of the American people.â€

â€œWe will use every means possible to make sure the constituents of these senators know that their elected representatives ignored them, and put Washington D.C. special interest politics over the effort to keep their own communities safer from the tragedy of gun violence,â€ they said.

The Manchin-Toomey background checks amendment allowed exemptions for private sales or gifts between families and friends and prohibited the creation of a national registry of guns. Polls show more than 80 percent of Americans support expanded background checks.

But the NRA had warned members the proposal would require checks for sales and gifts between family and friends and lead to a national registry.

The NRAâ€™s assertions are â€œa lie. That is simply a lie, and anybody who can read knows that is not factual,â€ Manchin, a strong gun-rights defender, said on the Senate floor on Wednesday.

Reid, a Nevada gun owner and gun-rights defender, questioned why his colleagues would reject a proposal backed by nearly 90 percent of Americans.

â€œWe must strike a better balance between the right to defend ourselves and the right of every child in America to grow up safe from gun violence,â€ Reid said.

Reid did not say after the vote whether the Senate would pursue any additional measures related to gun violence. An underlying bill on the Senate floor would tighten restrictions on gun trafficking and bolster school security, but that would probably also have a hard time gaining 60 votes.

The Senate also rejected several Republican-sponsored amendments backed by the NRA that would have expanded gun rights.

But the influence of the gun culture and the gun lobby was clear when an NRA-backed plan to allow gun owners with permits to carry concealed weapons across state lines also failed to reach the 60-vote threshold – but earned more votes, 57, than the background checks amendment.

Besides the â€œconcealed carryâ€ proposal, the Senate rejected on a 52-48 vote, eight short of the 60 needed, an amendment from Grassley that would have replaced the existing gun-control bill with a plan to focus on prosecuting gun crimes, improving mental health records for gun owners and funding better school safety measures.

â€œRather than restricting the rights of law-abiding Americans, we should be focusing on keeping guns out of the hands of violent criminals, which this legislation accomplishes,â€ Texas Republican Senator Ted Cruz said of Grassleyâ€™s measure.

(Additional reporting by Kim Dixon; Editing by David Lindsey and Peter Cooney)